Video projection systems are well known for projecting a composite colour image on a flat wall, rear screen or curved front screen surface. The image is generated via three coloured light beams (red, green and blue) which are centered and focused for convergence at a centre point of the screen.
Since the respective coloured light beams are projected at different angles for focusing at the centre point, mis-registration of the images is known to occur on the edges of the screen, etc.
In order to correct the problem of mis-registration of images in projection video systems, convergence systems have been developed for deflecting respective ones of the beams by predetermined amounts in accordance with information input to the convergence system by an operator using a cursor control, etc.
More recently, automatic convergence systems have been developed which utilize sensors for detecting mis-registration of a projected image at various zones on the screen and automatically deflecting the electron beams by the required amounts for obtaining convergence of the image without user input. The sensors in known automatic convergence systems have typically been fabricated using CCD arrays. The CCD arrays detect and digitize the projected image, which is then processed via microprocessor circuitry for detecting mis-registration. In response to detecting mis-registration, the microprocessor circuitry generates appropriate convergence coil signals, etc. A disadvantage of CCD based sensors is that such sensors integrate the image over an entire raster frame. Accordingly, the signal-to-noise ratio of the detected image relative to detected ambient light is very low. As a result, CCD image sensors are characterized by poor image detection, especially in relation to blue phosphor.
In order to overcome the disadvantages resulting from the use of CCD sensors, a number of prior art attempts have been made at using photodiode arrays for detecting the projected image. Because a photodiode array is peak sensitive, as opposed to time averaging as in CCD sensors, the intensely high peak intensities from a CRT phosphor can be readily sensed from low reflectivity surfaces such as rear screens and off-axis high-gain (i.e. curved) front screens.
For example, U.S. Pat. No. 4,642,529 (Sperry Corporation) discloses a photoresponsive sensor carried by an X-Y mount. The sensor is used for providing precise convergence measurement of beams in a CRT display by sensing the displacement of brightness peaks by an array of phosphorescent regions which are sequentially illuminated on the display screen. Convergence is calculated by calibrating the recorded brightness peaks as a function of beam displacement. The system is adapted to measure both vertical and horizontal components as well as line width.
U.S. Pat. No. 4,683,467 (Hughes Aircraft Company) discloses an image registration system in which a plurality of sensors are provided--one for each projected colour image. Each sensor comprises three photodetectors which are disposed in a right angle relationship for determining the position of a registration pattern on the screen. The degree of incidence of the registration pattern upon the photodetectors results in varying amounts of photocurrent generated thereby. By comparing the photocurrents produced by the horizontal and vertical photodetectors, different signals are produced, thereby indicating that the registration pattern is either in registration or out of registration.
U.S. Pat. Nos. 3,962,722 (Zenith Radio Corporation) and 4,441,120 (Philips Corporation) contemplate the use of quadrant photodetectors in a convergence system.
More particularly, the Zenith patent discloses a plurality of photocells which are preferably in a generally rectangular array. Similarly, the Philips patent discloses a square photosensitive cell consisting of four surface photodiodes placed in a quadrant for centering an electron beam on the crossing of the four diodes by comparing the signals generated by the photodiodes.
Although the above references discussed teach the use of quadrant photodiodes sensors in a convergence system, thereby overcoming some of the disadvantages of prior art CCD arrays, each of the prior art quad sensor configurations suffer from poor efficiency resulting from sensor dark current and ambient room light illuminating the sensor.
More particularly, since in many industrial applications the room lighting cannot be turned off, the sensors are exposed to considerable ambient light, thereby reducing the signal-to-noise detection ratio of the sensors. The colour blue is particularly difficult to distinguish in the presence of ambient light since the ambient light tends to "wash out" a blue image.
Furthermore, it is well known that a dark current is generated by photodiodes in proportion to the temperature operation of the photodiodes. Thus, thermally generated dark current within the prior art sensors further contributes to the reduced signal-to-noise detection ratio.